Touch screens are emerging as a popular means to interact with an electronic device. Touch screens can be mechanically mated with many different display types, such as cathode ray tubes (CRTs), liquid crystal displays (LCDs), plasma displays, electroluminescent displays, or displays used for electronic paper, such as electrophoretic displays. Many touch screens operate on the principle that when the screen is touched the touch changes an electrical property, such as capacitance or resistance, in a specific location of the touch screen. An electrical signal corresponding to the location of the touch can then be read in a controller unit, to control the operation of a device connected to, for instance, a display. Based on what electrical property is affected by the touch, such touch screens are commonly categorized as e.g. capacitive touch screens or resistive touch screens.
Such touch screens rely on one or more conductive transparent layers, commonly films such as indium tin oxide (ITO) thin-films, as part of an electrical circuit whose capacitance or resistance is modified in a specific location by the touch. The conductive transparent films in known touch screen structures are deposited on a supporting substrate which must be of suitable material to enable a transparent conductive film with good optical and electrical quality to be grown or deposited.
The display devices of the prior art are typically protected by a transparent layer from the viewing side of the display (see e.g. U.S. Pat. No. 5,688,551). These protective transparent layers may be of e.g. glass or of other material, suitable for mechanically and/or chemically protecting the display and/or for supporting a transparent electrode necessary for the operation of the display. As the thin transparent film of the touch screen requires a specific substrate to be deposited on, the touch screen is fabricated as a separate module which is added and aligned on top of the display module, to form a touch display. The separate manufacturing of the touch screen module enables a suitable substrate to be chosen for the transparent conductive film (or several films) of the touch screen.
In addition to the structural support provided by the substrate a conductive transparent film commonly also requires chemical and/or physical protection from one or both sides of the film. This type of encapsulation is required to protect the potentially sensitive transparent conductive film against, for instance, water and/or oxygen or against physical damage (e.g. scratching or bending). Thus the touch screen module adds additional layers through which the image of the display must be viewed.
Due to the added optical thickness of the touch screen module, touch screens, as implemented in touch displays of the prior art, significantly degrade the optical quality/usability of the touch display. This degradation is especially detrimental in touch displays used for e-paper, such as electrophoretic (EPD) displays, which are intended to mimic the appearance of a conventional paper. On displays used for e-paper a touch screen of the prior art destroys one of the key advantages of the display; that the image appears at the surface like in traditional paper and is thus easy and comfortable to view. This disadvantageous effect of conventional touch screen structures causes a particularly unpleasant appearance for the display from wide viewing angles, i.e. when the viewing direction is far away from the direction perpendicular to the plane of the display, and in conditions that would cause high glare and/or reflection in traditional emissive displays such as LCD-displays or OLED-displays. The traditional touch screen module solution, on the other hand, gives the user the sensation of reading the e-paper through a piece of glass which is uncomfortable and unnatural to the user.
Prior art discloses some structures which attempt to integrate the touch screen to a display. E.g. U.S. Pat. No. 5,852,487 discloses a resistive touch screen on a liquid crystal display (LCD), and U.S. Pat. No. 6,177,918 discloses a touch display having the touch screen fabricated on the same side of a common substrate with the display device.
Drawbacks of the structures disclosed in U.S. Pat. No. 5,852,487 include the strict requirements for the common substrate so that the substrate would enable electrode films with suitable optical and electrical properties to be fabricated on both sides of the common substrate. The publication even suggests an approach in which the substrate between the touch screen and the display is formed by laminating separate substrates for the touch screen and the display together, after the touch screen module and the display module have been separately fabricated on their dedicated substrates.
The structures disclosed in U.S. Pat. No. 6,177,918 on the other hand requires a specific arrangement between pixels of the display and the signal generating layer of the touch screen so that the display and the touch screen could be fabricated on the same side of a common substrate. Furthermore, strict material requirements for the substrates of the transparent conductive films still remain in the structures disclosed in this publication.
There exists a need for non-complicated reliable methods and device structures that allow a touch screen to be fabricated on a display such that the optical quality of the image and readability of the display are not compromised by the touch screen.